This invention concerns a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability and, more particularly, a Fexe2x80x94Cr alloy suitable for use in civil engineering and building structural materials requiring initial rust resistance, bending workability and toughness for weld zone.
As civil engineering and building structural materials, carbon steels such as SS 400 (JIS G 3101,JIS is Japanese Industrial Standard,here in after JIS) and high tensile steels such as SM 490(JIS G 3106) and such steel materials applied with painting or plating have mainly been used.
However, as designs for the materials have been varied, use of various kinds of materials have been studied in recent years.
Among them, since Fexe2x80x94Cr alloys which are excellent in corrosion resistance and aesthetic appearance scarcely require maintenance cost for rusting, they can be said to be highly attractive materials in view of life cycle cost (LCC).
Particularly, buildings constructed in coastal districts involve problems of short life and increased maintenance cost for suppressing corrosion. Further, also in view of the propagation for the water front development, the Fexe2x80x94Cr alloys have been greatly expected as corrosion resistant functional materials for use in civil engineering and building structures excellent in corrosion resistance, weldability and, particularly, initial rusting resistance.
Fexe2x80x94Cr alloys are generally classified in view of the metal structures into ferritic stainless steels represented by SUS 430 steels (JIS G 4304), martensitic stainless steels represented by SUS 410 steels (JIS G 4304) austenitic stainless steels represented by SUS 304(JIS G 4304), 2-phase stainless steels represented by SUS 329 steels (JIS G 4304) and precipitation hardened steels represented by SUS 630 (JIS G 4304).
Among various kinds of Fexe2x80x94Cr alloys described above, austenitic stainless steels which have been actually used most frequently having material strength, corrosion resistance, easy weldability, toughness of weld zone and general applicability have been studied, particularly, so far as the materials for use in civil engineering and building structures.
Such austenitic stainless steels have characteristics fully satisfying the characteristics required for civil engineering and building materials such as strength, corrosion resistance, fire resistance and toughness of weld zone.
However, such austenitic stainless steels
(1) contain a great amount of alloying elements such as Ni and Cr and, accordingly, are very expensive compared with carbon steels,
(2) cause stress corrosion cracking, and
(3) show greater heat expansion coefficient and relatively low heat conductivity compared with carbon steels, so that heat-affected strains upon welding tend to be accumulated and they are difficult to be used to materials requiring high accuracy. In view of the above, they involve a problem that it is difficult to apply them to the use of a general purpose structural materials in which carbon steels or carbon steels applied with painting or plating are used and their application range is restricted.
In view of the above, low Cr content alloy steels with the Cr content of 15 mass % or less have been studied recently for the application use to civil engineering and building materials as substitutes for plated or painted carbon steels. Application of the martensitic stainless steels in the field of the civil engineering and building materials is an example.
Since the Fexe2x80x94Cr alloys with the Cr content of 15 mass % or less have less Cr content and, further, less Ni content compared with Ni-containing Fexe2x80x94Crxe2x80x94Ni alloys as described above, they have a feature of being outstandingly inexpensive and having low heat expansion coefficient and high heat conductivity, as well as excellent in corrosion resistance and high yield strength compared with carbon steels.
Further, the martensitic stainless steels are also advantageous in that they are free from the worry of "sgr" embrittlement and 475xc2x0 C. embrittlement that are the problem in high Cr alloys containing 15 mass % or more of Cr and, further, free from the worry of stress corrosion cracking in chloride containing circumstances that gives a problem in austenitic stainless steels.
However, since the martensitic stainless steels represented by SUS 410 steels have C content as high as about 0.1 mass %, they are poor in the toughness of weld zone and the workability of the weld zone and require pre-heating upon welding to deteriorate the welding operationability, they still leave a problem in the application use to those materials requiring welding.
As a countermeasure for the problems described above, Japanese Patent Publication No. 13463/1976 for example, proposes a martensitic stainless steel for use in welding structures, containing 10 to 18 mass % of Cr, 0.1 to 3.4 mass % of Ni, 1.0 mass % or less of Si and 4.0 mass % or less of Mn in which C is reduced to 0.030 mass % or less and N is reduced to 0.020 mass % or less and massive martensitic structure is formed in the heat-affect zone, thereby improving the performance of the weld zone.
Further, Japanese Patent Publication No. 28738/1982 proposes a martensitic stainless steel excellent in the toughness of weld zone and workability, requiring neither pre-heating nor post heating before and after the welding by incorporating 10 to 13.5 mass % of Cr, 0.5 mass % or less of Si and 1.0 to 3.5 mass % of Mn, reducing C to 0.020 mass % or less and N to 0.020 mass % or less and, further, strictly restricting Ni to less than 0.1 mass %.
However, the techniques disclosed in Japanese Patent Publication Nos. 13463/1976 and 28738/1982 involve a problem that no countermeasure is taken for the problem inherent to the civil engineering and building structural materials as shown below.
When considering the application use to the civil engineering and building structures, those members such as pillars or beams are not exposed to severe circumstances after the completion of structures as outer wall materials. However, they are sometimes left in the outdoor in a short period of time of about several months after worked into structural members such as steel pipes or steel shapes with various sections in factories and shipping therefrom till the completion of the constructing operation for the structures. Accordingly, it is important to improve the initial rust resistance of the steel materials for suppressing occurrence of initial rust caused during construction period after shipping in view of the appearance, as well as in view of the durability of the structures after completion.
Further, when they are used as the civil engineering and building structural materials, since the requirement for the surface property is not so strict, it is desirable with an economical point of view that they can be used as hot rolled or hot rolled and annealed in a state where scales are not removed from the surface of steel plates.
Further, considering fabrication, for example, to steel shapes having various sections, there are great demand for the improvement of the toughness of the steel plate, particularly, elongation and bending workability in base steel plates or weld zones.
In view of the problems described above, Japanese Patent Laid-Open No. 302796/1999 proposes a hot rolled stainless steel sheet for use in building structures of excellent corrosion resistance, as well as a manufacturing method thereof, the steel having compositional ingredients comprising:
C: 0.005 to 0.1 mass %,
Si: 0.05 to 1.5 mass %,
Mn: 0.05 to 1.5 mass %,
P: 0.04 mass % or less,
S: 0.05 mass % or less,
Cr: 10 to 15 mass % and
N: 0.055 mass % or less, reducing (C+N) to 0.1 mass % or less and containing one or two of Ni and Cu within a range from 0.1 mass % or more and less than 1.0 mass %, with the balance of Fe and inevitable impurities.
Further, Japanese Patent Laid-Open No. 302797/1999 proposes a hot rolled stainless steel sheet for use in building structures, of excellent corrosion resistance, as well as a manufacturing method thereof, the steel having compositional ingredients comprising:
C: 0.005 to 0.1 mass %,
Si: 0.05 to 1.5 mass %,
Mn: 0.05 to 1.5 mass %,
P: 0.04 mass % or less,
S: 0.05 mass % or less,
Cr: 10 to 15 mass % and
N: 0.055 mass % or less, reducing (C+N) to 0.1 mass % or less and, further, containing one or two of Ni and Cu within a range from 0.1 mass % or more and less than 1.0 mass % with the balance of Fe and inevitable impurities and in which the average Cr content per one xcexcm in a surface metal layer of the hot rolled steel sheet is 7 mass % or more after mechanically peeling scales after hot rolling.
However, the techniques disclosed in Japanese Patent Laid-Open Nos. 302796/1999 and 302797/1999 merely utilize the technique of improving rust resistance by the addition of Ni and Cu, the effect of which has been known so far but gives no sufficient disclosure for the method of improving the initial rust resistance without deteriorating the toughness, particularly, the elongation and the bending workability in the base steel plate and the weld zone, and improvement therefor has been demanded.
In addition, as a method of improving the corrosion resistance, the weldability and the toughness of weld zone of the Fexe2x80x94Cr alloys, since enhancement of the purity and, in addition, addition of Nb or Ti for fixing carbon or nitrogen as carbides or nitrides are effective, various steels produced by using such means have been developed.
For example, Japanese Patent Laid-Open No. 13060/1985 discloses a stainless steel intended for the improvement of corrosion resistance by adding Nb as a stabilizing agent for carbon and nitrogen in an appropriate amount and further shows that the corrosion resistance can be improved further by the addition of Mo, Ni and Cu.
However, there is no sufficient study on the technique of effectively improving the toughness of weld zone and particularly the initial rust resistance for the duration from shipping to construction, for in the application of building structural materials and it has been demanded to establish a further improved method in addition to the existent technique of adding alloying elements such as Cu, Ni, Mo, Ti and Nb or reducing C and N known so far as described above.
This invention has been developed in view of the foregoing situations and is directed to a Fexe2x80x94Cr alloy having not only excellent weldability, corrosion resistance and workability but also excellent initial rust resistance.
A first aspect of this invention is a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability, and having a composition comprising:
C: more than about 0.0025 mass % and less than about 0.03 mass %,
N: more than about 0.0025 mass % and less than about 0.03 mass %,
Si: more than about 0.1 mass % and less than about 2.0 mass %,
Mn: more than about 0.1 mass % and less than about 3.0 mass %
Cr: more than about 8.0 mass % and less than about 15 mass %,
Al: less than about 0.5 mass %,
P: less than about 0.04 mass %,
S : less than about 0.03 mass %,
Ni: from about 0.01 mass % to about 3.0 mass %,
Co: from about 0.01 mass % to about 0.5 mass %,
V: from about 0.01 mass % to about 0.5 mass % and
W: from about 0.001 mass % to about 0.05 mass %, and a X value in the following equation (1), satisfies: Xxe2x89xa611.0, the balance substantially being Fe and inevitable impurities.
X value=Cr(mass %)+Mo(mass %)+1.5Si(mass %)+0.5 Nb(mass %)+0.2V(mass %)+0.3 W(mass %)+8 Al(mass %)xe2x88x92Ni(mass %)xe2x88x920.6 Co(mass %)xe2x88x920.5 Mn(mass %)xe2x88x9230 C(mass %)xe2x88x9230 N(mass %)xe2x88x920.5 Cu(mass %)xe2x80x83xe2x80x83(1) 
A second aspect of this invention is a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability as defined in the first aspect described above and having a Z value shown by the following equation (2) can satisfy: 0.03xe2x89xa6Z valuexe2x89xa61.5.
Z value=(Co(mass %)+1.5V(mass %)+4.8 W(mass %))xe2x80x83xe2x80x83(2) 
A third aspect of this invention is a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability as defined in the first or second aspect described above and wherein C/Nxe2x89xa60.60.
A fourth aspect of this invention is a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability as defined in the first, second or third aspect described above, wherein the alloy has a composition containing at least one element selected from:
Cu: from about 0.0001 mass % to about 3.0 mass % and
Mo: from about 0.0001 mass % to about 3.0 mass %.
A fifth aspect of this invention is a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability as defined in the first, second, third or fourth aspect described above, wherein the alloy has a composition containing at least one element selected from:
Ti: from about 0.0001 mass % to about 0.7 mass %,
Nb: from about 0.0001 mass % to about 0.7 mass %,
Ta: from about 0.0001 mass % to about 0.7 mass % and
Zr: from about 0.0001 mass % to about 0.5 mass %.
A sixth aspect of this invention is a Fexe2x80x94Cr alloy having excellent initial rust resistance, workability and weldability as defined in the first, second, third, fourth or fifth aspect described above, wherein the alloy has a composition containing B: from about 0.0002 mass % to about 0.002 mass %.